An enlargement lens system using a light guide device with a light surce of an enlarger for illuminating a stop-value scale thereof is generally known. It is used to simplify the photographic enlargement operation in a dark room. As well known in such a method of illuminating a stop-value scale for an enlargement lens system, a light ejected from the enlarger into a gap formed between an outer peripheral portion of a rear lens group frame and a lens mount thereof is reflected radially at a light guide disposed inside a stop-value scale member. This will achieve stop-value scale illumination.
However, this techinique of stop-value scale illumination has a material disadvantage when used in an enlargement with picture trimming or when applied to a enlargement lenses having a long focal length and a large diameter. Specifically, in an enlargement of a picture, trimming of an original picture to properly frame the sections of interest is often carried out. The trimming operation is accomplished by use of light shut-off plate installed in the enlarger. Therefore, in the case of the above-described stop-value scale illuminating, since the transmitting light will pass through the gap formed by the outer peripheral portion and the lens mount, then light around the margin or peripheral light is partially shut off by the light shut-off plate, i.e., the light for illuminating the stop-value scale is cut away. As a result, illumination of the stop-value scale is not achieved.
Also, it is required in the enlargement process to have the time duration of light-exposure minimized. Additionally, a general requirement exists that the enlargement lens have a large diameter for easy handling thereof. Generally, large diameter lenses tend to deteriorate in performance in situations involving light from the margin portions of the picture. For this reason, non-uniform light exposure during the enlargement operation results. Therefore, a technique is employed to increase the effective diameter of a lens to prevent deterioration of the marginal light. As a result, however, the rear lens frame of the large diameter lens is increased and a gap is formed by the peripheral portion of the rear lens group and the lens mount. This gap is extremely small and it is very difficult to pass transmitting light necessary for illuminating the stop-value scale through this gap.
Additionally, in an enlargement lens having a long focal length where the rear lens frame is large with respect to the inner diameter of the mount portion, the stop-value scale illumination technique is not practiced.